Sorry to hear about this. Seems to becoming more common as the days, weeks, months, and years go by. "I bet they will hinge it" on the one I have in bold. Maybe a tree hugger saw someone cutting into the grass and let out a blood curdling scream.... I hope you can resolve this with the Parks Director. Best of luck.
For the record, my town has basically the same language as below and if they wanted, they could slam the brakes on metal detecting anytime they choose.
Park Rules
Parks are open to the public every day of the year from 6:00 a.m. to 11:00 p.m., unless otherwise posted.
Vehicles in the parks are restricted to roads. The speed limit is 25 mph, unless otherwise posted.
Ride bicycles on a paved vehicular road, or path designated for that purpose. A bicyclist is permitted to wheel or push a bicycle by hand over any grassy area or wooded trail or on any paved area reserved for pedestrian use.
Swimming, bathing and wading may take place only in places provided for these activities.
Dogs and other pets must be on a leash and under the control of the owner. No pets are allowed on playgrounds or courts at any time.
Place trash in trash receptacles only. If trash receptacles are not provided, take your refuse with you. Never dump or deposit trash on any park lands or in park waters.
Cooperate in keeping the restrooms and washrooms in a neat and sanitary condition.
Do not set fires except in designated cooking areas. Picnickers must completely extinguish all fires before leaving the park. Never throw lighted matches, burning cigarettes or other flammable material in any park area.
Alcoholic beverages are prohibited in the parks, except by special permission of the Director.
Never damage, pollute, disfigure or remove any natural resource, structures or equipment in the parks. Do not hunt, kill, molest, frighten or tease any animals, reptiles or birds in the park.
Bottles, containers or other items made of glass are prohibited from the park.
Motorized model airplanes are not allowed in the parks.
No soliciting, vending, advertising or sign posting is allowed in the parks, except by special permission of the Director.